1. Field of the Invention
This invention generally relates to electric lamps and has particular reference to an improved high-pressure metal-halide discharge lamp having integral means for modifying the color-rendering characteristics and chromaticity of its light output.
2. Description of the Prior Art
High-pressure discharge lamps which utilize mercury and various metal-halides in the arc tube to provide an efficient light source are disclosed in U.S. Pat. No. 3,234,421 issued Feb. 8, 1966 to G. H. Reiling. A lamp of this type which provides a light output of improved color by dosing the arc tube with mercury, a halogen such as iodine, an alkali metal such as sodium, and scandium (either alone or together with thorium) is described in U.S. Pat. No. 3,407,327 issued Oct. 22, 1968 to F. Koury et al.
According to a more recent improvement in the art, the spectral emission of a high-pressure mercury discharge lamp is modified to provide a light output which is similar to that of an incandescent lamp by first coating the inner surface of the outer protective envelope with a non-luminescent material (such as silica, titania, magnesia, alumina or mixtures thereof) and then with a phosphor mixture containing selected amounts yttrium vanadate phosphor (or yttrium phosphate-vanadate phosphor) and a second phosphor such as magnesium fluoro-germanate or magnesium arsenate phosphor. A lamp of this type is disclosed in U.S. Pat. No. 3,825,792 issued July 23, 1974 to F. Rokosz et al.
Color-correction of a high-pressure mercury vapor lamp by blending a green-emitting phosphor (strontium chlorosilicate activated by divalent europium) with a red-emitting phosphor (such as yttrium vanadate phosphate activated by europium) and coating the inner surface of the protective envelope with the resulting mixture to convert ultraviolet radiations into visible radiations of the respective colors is disclosed in U.S. Pat. No. 3,866,083 issued Feb. 11, 1975 to R. K. Datta et al.
While metal-halide discharge lamps, especially those employing sodium and scandium in the arc tubes, provide important advantages in commercial lighting applications since their efficacy is higher than that of conventional mercury-vapor lamps, the spectral distribution of their light output is such that the color of articles illuminated by these lamps is distorted to such an extent that the lamps cannot be used for lighting department stores and the like where color rendition is very important to the customer as well as successful merchandising. It would accordingly be very desirable to provide a light source having the high efficacy of a metal-halide type discharge lamp but a source-color and color-rendition characteristics that are similar to those of an incandescent lamp.